Metallic coating method

ABSTRACT

Method for applying a metallic coating to a ferrous base metal strand wherein the surface of the strand is first thoroughly cleaned to render it receptive to a molten coating metal. During the cleaning step, the strand may be heated to a temperature less than the melting point of the metallic coating metal. A nonconductive coating pot is provided which has a primary coil within its side walls. Power is supplied to the primary coil which induces heavy secondary currents in the coating metal within the pot. This current is converted into heat by resistance of the coating metal itself, thereby supplying a high quantity of heat directly in the coating metal. The induced secondary current constantly stirs the molten coating metal so as to help provide a bright bath surface where the strand exits from the bath, and minimizes accumulation of oxides and dross.

United States Patent [1 1 Schwieterman 1 1 METALLIC COATING METHOD [75]Inventor: Roman A. Schwieterman,

Middletown, Ohio [22] Filed: Dec. 20, 1972 [21] Appl. No.: 316,826

[451 June 3,1975

Primary Examiner-Leon D. Rosdol Assistant Examiner-Edith L. RollinsAttorney, Agent, or FirmMelvil1e, Strasser, Foster & Hoffman [5 7]ABSTRACT Method for applying a metallic coating to a ferrous base metalstrand wherein the surface of the strand is first thoroughly cleaned torender it receptive to a molten coating metal. During the cleaning step,the strand may be heated to a temperature less than the melting point ofthe metallic coating metal. A nonconductive coating pot is providedwhich has a primary coil within its side walls. Power is supplied to theprimary coil which induces heavy secondary currents in the coating metalwithin the pot. This current is converted into heat by resistance of thecoating metal itself, thereby supplying a high quantity of heat directlyin the coating metal. The induced secondary current constantly stirs themolten coating metal so as to help provide a bright bath surface wherethe strand exits from the bath, and minimizes accumulation of oxides anddross.

5 Claims, 2 Drawing Figures [52] US. Cl 427/45; 219/1065 [51] Int. Cl.1305c 3/0; B44d 1/20 [58] Field of Search 117/71 M, 114 R, 114 A,117/114 B, 114 C, 93.2,113;148/6.11, 6.14 R; 13/27, 26 X [56] ReferencesCited UNITED STATES PATENTS 2,110,893 3/1938 Sendzimir 117/71 M2,322,618 6/1943 DeMare. 13/26 2,643,201 6/1953 Chadsey et a1. 117/93.22,698,810 1/1955 Stauffer 117/93.2 2,824,021 2/1958 Cook et a1. 117/114A 3,203,824 8/1965 McQuaid et a1. 117/114 R 3,478,156 11/1969 Segsworth13/27 3,508,977 4/1970 Basche 148/6.11 3,666,537 5/1972 Williams 117/114C FOREIGN PATENTS OR APPLICATIONS 849,503 9/1952 Germany 117/114 CMETALLIC COATING METHOD BACKGROUND OF THE INVENTION This inventionrelates to the hot dip coating of a ferrous base metal strand. Suchprocesses have advantageously been used for many years in the productionof iron or steel strip having a thin coating of zinc, aluminum, terne,and various other combinations.

Generally considered, the processes of the prior art all involve as afirst step the thorough cleaning of the surface of the strand to becoated. This cleaning operation may be carried out by successiveoxidizing and reducing heat treatments as taught by Sendzimir, bychemical cleaning, or by various other processes.

The clean strand is then passed into a bath of molten coating metal, andwithdrawn in a generally upwardly path of travel. The molten coatingmetal adhering to the surface of the strand and drawn upwardly from thepath is then finished by means of coating rolls, air knives, or thelike, and the molten coating is subsequently solidified. According tothe prior art, the molten coating metal bath is generally maintained inan externally heated iron pot. Long experience with these coating metalpots has disclosed several disadvantages, particularly in the case ofpots for molten aluminum. First of all, the pot has a relatively shortlife. This short life is caused by several factors, including the rapidbuild-up of dross on the pot bottom, and creep or bulging of the wallscaused by high temperature of the externally applied heat and the weightof the coating metal contained,

In addition, the quantity of heat which can be supplied externally is ofcourse limited. The prior art has found it necessary with coating potsof this type to heat the strip to a temperature in excess of the meltingpoint of the coating metal prior to passing the strip into the bath. Inother words, enough heat could not be supplied to the coating metal bathto maintain the coating metal molten and at the same time heat arelatively cool (cool with respect to the melting point of the coatingmetal) strand up to coating temperature.

Finally, the large, uncovered surface of molten coating metal in the potleads to a rapid formation of oxides and dross at the surface. Thisaccumulation of oxides and dross at the surface of the bath is one ofthe most significant problems encountered today in the hot dip coatingprocesses. That is, upon emergence from the bath, the strand tends topick up particles of dross and oxide from the surface of the bath,resulting in heavy edges or other imperfections in the coating applied.

Keeping the foregoing comments in mind, it is an object of thisinvention to provide an improved method for applying a metallic coatingto a ferrous base metal strand.

More specifically, one object of this invention is to provide a hot dipcoating method wherein the base metal strand can enter the bath at atemperature below the melting point of the coating metal.

Another object of the invention is to provide a hot dip coating methodwhich can generate sufficient heat within the coating metal bath thatadditional coating metal may be melted directly in the pot.

Still another object of the invention is to provide a hot dip coatingmethod which will greatly enhance coating pot life.

Still a further object of this invention is to provide a hot dipmetallic coating method which permits very accurate control over thetemperature throughout the molten coating metal bath.

Still a further object of the invention is the provision of a metalliccoating method wherein the accumulation of dross and oxides in the bathare minimized.

A further object of this invention is to provide a coating method whichmaintains a bright, oxide and dross free bath surface at the point wherethe base strand exits the coating metal bath.

SUMMARY OF THE INVENTION Broadly considered, this invention relates tothe hot dip coating of a ferrous base metal strand with one of theconventional metallic coating metals. The process includes the steps ofthoroughly cleaning the surface of the strand to render it receptive toa molten coating metal. The cleaned strand is then passed into a bath ofthe molten coating metal. The bath is maintained in a vessel or potconstructed of a non-conductive material having a primary coil withinits side walls. Power will be applied to the primary coil which willinduce secondary currents in the charge of coating metal in the pot. Thesecondary currents are converted to heat by the resistance of the chargeitself.

The size of the pot and the induced secondary currents serve tocontinuously agitate the molten coating metal in the bath so as toprevent dross accumulation on the bottom of the pot, and help provide abright bath surface where the coated strand exits from the bath.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of acoating method according to this invention.

FIG. 2 is a schematic diagram showing the coating pot utilized in themethod of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, acomplete coating process embodying the teachings of this invention hasbeen illustrated schematically. A coil of a suitable ferrous base metalstrand is indicated at 10. The continuous strip or sheet passes from therolls 11 and 12 as indicated to enter the top of the first furnacesection 14. This first section of the furnace 14 may be of the directfired, non-oxidizing type. That is, there is approxi mately a 5 percentexcess of combustibles introduced into this section. Furnace temperaturemay be on the order of 2300F., so that the base metal strand will berapidly heated to a temperature on the order of at least 1 F. This iseffective to almost instantaneously burn surface contaminants such asoil and the like from the surface of the strip. The verticalconfiguration is desirable in that it eliminates the necessity forsupporting rollers in the hot sections of the furnace.

The second section of the furnace indicated generally at 16 may be ofthe radiant heating type. In this section of the furnace, thetemperature of the base strand will be raised to a temperature on theorder of 1350F. to 1550F., reaching its maximum temperature at the point18. A reducing atmosphere will be supplied to this portion of thefurnace, as well as to the succeeding portions of the furnace describedbelow.

The third section of the furnace indicated generally at 20 is a tubecooling zone.

The final section of the furnace 22 may include means for jet cooling ofthe strand, in some cases to a temperature below the melting point ofthe coating metal being used.

The strip passes out of the furnace portion 22, over the turn down roll24 and through the snout 26 into the bath of molten coating metalindicated generally at 28. This bath is explained in more detailhereinafter and shown in FIG. 2.

The strip is withdrawn from the bath 28 in a generally vertical path oftravel past the jet finishing knives indi cated generally at 30, andafter allowing time for solidification, over the turning roll 32 andcoiled for storage and shipment as at 34.

Portions of the process described are entirely conventional and wellknown in the art. For example, the furnace configuration is known perse. Similarly, the jet finishing techniques are known per se. Theprimary aspect of the method of this invention centers about the bath ofmolten coating metal 28 which will be described hereinafter. One veryimportant advantage of the particular bath arrangement to be describedis that the base metal strand may be cooled in the furnace portions 20,22, and in the snout 26 to a temperature below the melting point of thecoating metal. In other words, in the case of aluminum, the strip may becooled to a temperature on the order of l200 prior to entering themolten metal bath. The utilization of lower strip temperatures uponentering the bath has been found to reduce substantially the interfacealloy formation; that is, the alloy formation at the interface betweenthe base strand and the coating metal. Of course, the reduction inthickness of alloy formation greatly improves adherence of the coatingmetal.

Turning now to FIG. 2, the molten metal bath of this invention will bedescribed in more detail. The molten metal bath is maintained in the potindicated generally at 40. Hereinafter, the pot 40 will be referred toas a coreless induction coating." During use, the coreless inductioncoating pot 40 will be filled with molten metal to the level 42.

As previously indicated, the base metal strand to be coated passesthrough the snout 24, over the turn down roll 26, and into the moltenmetal bath. It will be observed that the lower end of the snout 24a isimmersed in the bath of molten coating metal, so that the properatmosphere (i.e. a reducing or non-oxidizing atmosphere) may bemaintained at all times within the snout 24.

Suitably mounted for rotation with in the molten metal bath are the potrolls 44 and 46, and the stabilizer roll 48. These aspects of thecoating machinery are entirely conventional.

The coreless induction coating pot indicated generally at 40 includes aninterior wall and bottom section of refractory material indicated at 50.This interior structure may be formed in a variety of ways. For example,an exemplary pot in commercial use includes two layers of ceramic brick.These bricks are precision laid with thin joints between adjacentbricks. On the inside of the two layers of ceramic brick may be one ormore layers of a ceramic feltmaterial. The innermost layer may be alayer of ceramic grout material on the order of one quarter inch thick.As previously indicated, a variety of materials may be used for theinterior pot portions 50. It is important that whatever material is usedbe electrically non-conductive and that the material be chosen forcompatability under high temperature with the molten coating metal.

Surrounding the interior wall portion 50 will be the primary inductioncoil indicated generally at 52. The primary coil may be constructed ofwater cooled copper tubing. The coil 52 will be connected in anysuitable manner to a source of power.

Cooling coils 53a and 5312 will preferably be provided above and belowthe primary coil 52. These coils are used and controlled to keeptemperatures in the wall portion 50 evenly dispersed, and preventcracking and spalling of the pot walls.

Surrounding the primary coil 52 will be the structural support for thecoreless induction melting pot indicated generally at 54. Thisstructural framework may be of steel or the like, and will provide thestrength necessary to retain the large quantity of molten metal withinthe pot.

If desired, the pot can be mounted on wheels 56 which in turn operate onthe track 58. By this expedient, a plurality of coating pots may beinterchanged with the rest of the coating equipment. For example, potscontaining different coating metals such as aluminum and zinc may beused interchangeably.

In operation of the coating pot, alternating current at a frequency of60 cycles will be transmitted over watercooled cables to the primarycoil 52 described earlier. The power applied to this primary coilcreates a magnetic flux which passes through the material within thepot. The material in the pot acts as the secondary winding of atransformer having a single turn.

The high density, rapidly changing magnetic flux generated by theprimary coil induces heavy secondary currents in the material in thepot. These heavy secondary currents are converted into heat by theelectrical resistance of the material in the pot.

These induced secondary currents provide a continuous stirring oragitation of the molten material in the pot as indicated by the arrows60. This stirring action is extremely important to the method of thisinvention. First of all, commercial practice according to the teachingsof this application has established that the stirring action will helpmaintain a bright, oxide and dross free bath surface at the point wherethe strip exits the coating metal bath. The induced currents at the bathsurface flow from the center radially to the periphery and are believedto create an additive effect to the washing action of the jet nozzles tokeep oxides formed in jet finishing pushed away from the strip, therebymaintaining a bright area around the strip. The maintaining of a brightbath surface in this area virtually eliminates the pickup of oxides andresulting coating imperfections encountered with conventional processes.

Secondly, this stirring action apparently maintains oxide and dross in asuspension uniformly distributed throughout the coating pot. This ofcourse substantially eliminates the build up of oxides and dross at thebottom of the pot. Prior experience with aluminum would lead one toexpect a significant and troublesome oxide build up on the pot wall in aband at mid coil elevation. Oxide build up has been insignificantprobably because of complex current patterns caused by high diameter/-depth ratio and submerged pot equipment.

The coreless induction pot design just described has several veryimportant additional advantages in a metallic coating operation. Firstof all, it will be apparent that the heat is generated within the potitself. This permits very accurate temperature control of the moltenmaterial in the pot.

Secondly, this permits the more rapid generation of heat in the pot.This, in turn, provides at least two important advantages. It willpermit the melting of additional coating metal directly in the pot. Thatis, pigs of solid coating metal 62 may be transported by the conveyor orchute 64 directly into the coating pot. They will be maintained in asingle location in the pot by the baffle plate 66. The deflector plate68 will prevent molten metal from splashing directly out of the pot.

In addition, the rapid generation of heat within the coating pot is whatin fact makes it possible for the strip to enter the pot at atemperature below melting point of the coating metal. Sufficient heatcan be generated within the pot to raise the strip to coatingtemperature without causing coating metal to freeze on the surface ofthe strip at entry.

The coreless induction pot of this invention embodies what might becalled a short coil design. That is, the ratio between the diameter ofthe pot and the depth of the pot is very different from previously knowncoreless induction furnaces. In the commercial unit referred to earlier,the pot utilized has a bath diameter of ten feet and a depth of ninefeet.

It is believed that the foregoing constitutes a full and completedisclosure of this invention, and no limitations are intended exceptinsofar as specifically set forth in the claims that follow.

The embodiment of the invention in which an exclusive property orprivilege is claimed are defined as follows,

1. In a hot dip metallic coating method including the steps ofthoroughly cleaning the surface of a ferrous base metal strand toprepare said surface to be wet by the coating metal, passing saidcleaned base metal strand into a bath of molten coating metal,withdrawing said strand from said bath in an upward path of travelwhereby a quantity of said molten coating metal will be carried fromsaid bath by said strand, finishing said molten metal adhering to saidstrand, and solidifying said molten coating; the improved steps of:

a. providing a nonconductive coreless induction coating pot forcontaining said bath of molten coating metal;

b. providing a primary coil within the side walls of said pot; and

c. supplying power to the primary coil of said coreless induction potwhereby to induce heavy secondary currents in the coating metal in saidpot which are converted into heat by the resistance of said coatingmetal, said induced secondary currents serving to continuously agitatethe molten coating metal whereby to help provide a bright bath surfacewhere said strand exists from said bath and to substantially eliminatethe accumulation of oxides and dross on the bottom of said pot and tominimize the accumulation of oxides and dross on the walls of said pot.

2. The method claimed in claim 1 wherein the depth of said corelessinduction coating pot is less than the diameter of said corelessinduction coating pot.

3. In a hot dip metallic coating method including the steps ofthoroughly cleaning the surface of a ferrous base metal strand toprepare said surface to be wet by the coating metal, passing saidcleaned base metal strand into a bath of molten coating metal,withdrawing said strand from said bath in an upward path of travelwhereby a quantity of said molten coating metal will be carried fromsaid bath by said strand, finishing said molten metal adhering to saidstrand, and solidifying said molten coating; the improved steps of:

a. providing a non-conductive coreless coating pot;

b. supplying said pot with a charge of the desired coating metal;

c. providing a primary coil within the side walls of said pot; and

d. supplying power to said primary coil of said pot whereby to induceheavy secondary currents in said charge of coating metal in said pot,said secondary currents being converted into heat by the resistance ofsaid charge and serving to continuously agitate the molten coating metalwhereby to provide a bright bath surface where said strand exits saidbath and to minimize the formation and accumulation of oxides and dross.

4. The method claimed in claim 3 including the step of cooling sasidstrand to a temperature less than the melting point of said metalliccoating after said cleaning of said strand and prior to the passing ofsaid strand into said bath.

5. The method claimed in claim 1 wherein the coating metal is aluminum.

1. IN A HOT DIP METALLIC COATING METHOD INCLUDING THE STEPS OFTHOUROUGHLY CLEANING THE SURFACE OF A FERROUS BASE METAL STRAND TOPREPARE SAID SURFACE TO BE WET BY THE COATING METAL, PASSING SAIDCLEANED BASE METAL STRAND INTO A BATH OF MOLTEN COATING METAL,WITHDRAWING SAID STRAND FROM SAID BATH IN AN UPWARD PATH OF TRAVELWHEREBY A QUANTITY OF SAID MOLTEN COATING METAL WILL BE CARRIED FROMSAID BATH BY SAID STRAND, FINISHING SAID MOLTEN METAL ADHERING TO SAIDSTRAND, AND SOLIDIFYING SAID MOLTEN COATING; THE IMPROVED STEPS OF: A.PROVIDING A NONCONDUCTIVE CORELES INDUCTION COATING POT FOR CONTAININGSAID BATH OF MOLTEN COATING METAL; B. PROVIDING A PRIMARY COIL WITHINTHE SIDE WALLS OF SAID POT; AND C. SUPPLYING POWER TO THE PRIMARY COILOF SAID CORELESS INDUCTION POT WHEREBY TO INDUCE HEAVY SECONDARYCURRENTS IN THE COATING METAL IN SAID POT WHICH ARE CONVERTED INTO HEATBY THE RESISTANCE OF SAID COATING METAL, SAID INDUCED SECONDARY CURRENTSSERVING TO CONTINUOUSLY AGITATE THE MOLTEN COATING METAL WHEREBY TO HELPPROVIDE A BRIGHT BATH SURFACE WHERE SAID STRAND EXISTS FROM SAID BATHAND TO SUBSTANTIALLY ELIMINATE THE ACCUMULATION OF OXIDES AND DROSS ONTHE BOTTOM OF SAID POT AND TO MINIMIZE THE ACCUMULATION OF OXIDES ANDDROSS ON THE WALLS OF SAID POT.
 1. In a hot dip metallic coating methodincluding the steps of thoroughly cleaning the surface of a ferrous basemetal strand to prepare said surface to be wet by the coating metal,passing said cleaned base metal strand into a bath of molten coatingmetal, withdrawing said strand from said bath in an upward path oftravel whereby a quantity of said molten coating metal will be carriedfrom said bath by said strand, finishing said molten metal adhering tosaid strand, and solidifying said molten coating; the improved steps of:a. providing a nonconductive coreless induction coating pot forcontaining said bath of molten coating metal; b. providing a primarycoil within the side walls of said pot; and c. supplying power to theprimary coil of said coreless induction pot whereby to induce heavysecondary currents in the coating metal in said pot which are convertedinto heat by the resistance of said coating metal, said inducedsecondary currents serving to continuously agitate the molten coatingmetal whereby to help provide a bright bath surface where said strandexists from said bath and to substantially eliminate the accumulation ofoxides and dross on the bottom of said pot and to minimize theaccumulation of oxides and dross on the walls of said pot.
 2. The methodclaimed in claim 1 wherein the depth of said coreless induction coatingpot is less than the diameter of said coreless induction coating pot. 3.In a hot dip metallic coating method including the steps of thoroughlycleaning the surface of a ferrous base metal strand to prepare saidsurface to be wet by the coating metal, passing said cleaned base metalstrand into a bath of molten coating metal, withdrawing said strand fromsaid bath in an upward path of travel whereby a quantity of said moltencoating metal will be carried from said bath by said strand, finishingsaid molten metal adhering to said strand, and solidifying said moltencoating; the improved steps of: a. providing a non-conductive corelesscoating pot; b. supplying said pot with a charge of the desired coatingmetal; c. providing a primary coil within the side walls of said pot;and d. supplying power to said primary coil of said pot whereby toinduce heavy secondary currents in said charge of coating metal in saidpot, said secondary currents being converted into heat by the resistanceof said charge and serving to continuously agitate the molten coatingmetal whereby to provide a bright bath surface where said strand exitssaid bath and to minimize the formation and accumulation of oxides anddross.
 4. The method claimed in claim 3 including the step of coolingsasid strand to a temperature less than the melting point of saidmetallic coating after said cleaning of said strand and prior to thepassing of said strand into said bath.